Obligate biotrophs are pathogens that establish intimate parasitic relationships with the host that they infect. Often times these relationships involve some kind of modification or reprogramming of the host cell(s) to accommodate the pathogen's subsequent growth and development. Plant-parasitic nematodes are obligate biotrophs that mainly attack the roots of plants and cause over $100 billion in crop damage annually (Sasser and Freckman, 1987). The most economically important plant-parasitic nematodes include the cyst forming nematodes of Heterodera and Globodera spp. These sedentary endoparasitic nematodes form intimate parasitic relationships with their hosts by penetrating the root as motile juveniles and migrating intracellularly until they reach the root vasculature where they select a single cell to initiate a feeding site. The initial syncytial cell undergoes developmental changes to re-differentiate into a syncytium to support subsequent nematode growth and development in later sedentary stages (Davis et al., 2004). The syncytium forms when neighboring cells fuse as a result of partial cell wall degradation (Endo, 1964), creating a permanent feeding cell that shares characteristics with plant cell types including meristematic cells, endosperm cells, transfer cells, and developing xylem (Mitchum et al., 2008). It has been proposed that the development and maintenance of the syncytium is dependent on the secretory effector proteins originating in the esophageal gland cells and delivered into the host root through the stylet of plant-parasitic nematodes (Davis et al., 2008). Recently, the cyst nematode secreted CLAVATA3/ESR(CLE)-like effector proteins have been shown to act as ligand mimics of plant CLE peptides, and are required for successful nematode infection (Wang et al., 2005; Patel et al., 2008; Lu et al., 2009; Wang et al., 2010a; Wang et al., 2010b).
Plant CLEs are small peptide ligands involved in regulating a population of specialized cells, called stem cells, which allow postembryonic organogenesis to occur (Simon and Stahl 2006). These stem cell pools can be found in the shoot apical meristem (SAM), the root apical meristem (RAM), and the vascular cambium. Whether or not these stems cells remain in an undifferentiated state or differentiate into new plant tissues is tightly controlled by CLE signaling pathways. In Arabidopsis, the population of stem cells which resides in the organizing center (OC) of the SAM is maintained by the expression of the transcription factor WUSCHEL (WUS) (Laux et al., 1996). Differentiation of those stems cells is promoted when the ligand-receptor pair of CLAVATA3 (CLV3), a small extracellular peptide ligand in the CLE family (Fletcher et al., 1999; Rojo et al., 2002), binds to CLV1 (Ogawa et al., 2008), a leucine-rich-repeat receptor like kinase (LRR-RLK) and downregulates WUS. Previous models have suggested that CLV1 forms a receptor complex with the LRR-receptor like protein (RLP) CLV2 (Clark et al., 1993; Kayes and Clark, 1998; Jeong et al., 1999; Trotochaud et al., 1999). More recently, it has been suggested that CLV1 acts in parallel or together with the heterodimer receptor complex of CLV2 and CORYNE (CRN) (Miwa et al., 2008; Muller, 2008; Bleckmann et al., 2010; Zhu et al., 2010). In comparison to the SAM, much less is known about the regulation of the stem cells in the RAM. The quiescent center (QC) is the equivalent to the OC in the SAM. However, there are significant differences between the OC and the QC. In contrast to the OC, the cells surrounding the QC are maintained as stem cells. In addition, stem cells are differentiated in both proximal and distal directions. This indicates that there is a signaling ligand involved in cell-cell communication to maintain the cells surrounding the QC as stem cells, and a signal to promote differentiation (Sarkar et al., 2007; Stahl et al., 2009). Previous reports have identified that the WUS-related homeobox 5 (WOX5) transcription factor is expressed in the QC of the RAM and is required to maintain the distal stem cell pool (Sarkar et al., 2007). Recently it has been shown that CLE40, the closest homolog to CLV3, is expressed in the columella cells and regulates expression of WOX5 (Stahl et al., 2009). The WOX5/CLE40 signaling pathway appears to only control the distal stem cell pool, indicating that other CLE signaling pathways may exist to control the proximal stem cell pool. Consistent with these observations, a number of Arabidopsis CLEs are expressed in roots (Sharma et al., 2003), and when some of these CLEs are overexpressed they have been shown to cause premature termination of the primary root meristem (Fiers et al., 2004; Strabala et al., 2006; Meng et al., 2010). In addition, the short root phenotype has been shown to be dependent on CLV2 and CRN perception (Casamitjana-Martinez et al., 2003; Fiers et al., 2005; Miwa et al., 2008; Meng et al., 2010). Taken together this indicates that a CLV-like and CLE-controlled signaling pathway can act in the root.
CLE-like genes from nematodes have been reported in the soybean cyst nematode (SCN, H. glycines) (Wang et al., 2005; Wang et al., 2010a), the beet cyst nematode (BCN, H. schachtii) (Patel et al., 2008; Wang et al., 2010b), and the potato cyst nematode (PCN, G. rostochiensis) (Lu et al., 2009). BCN CLEs have been detected in the dorsal gland ampulla indicating they are likely secreted from the stylet into host cells (Patel et al., 2008). More recently, SCN CLEs have been shown to be secreted directly to the syncytial cytoplasm where the variable domain is thought to redirect the nematode CLE peptides to the apoplast (Wang et al., 2010a). These findings suggest that when delivered to the apoplast, nematode CLEs would be available to interact with extracellular receptors to function as ligand mimics of plant CLE signaling pathways. Overexpression studies have shown that nematode CLEs can trigger plant CLE signaling pathways (Wang et al., 2005; Lu et al., 2009; Wang et al., 2010a; Wang et al., 2010b), but the identity of the receptors and downstream signaling pathways that are activated to initiate developmental cascades required for the re-differentiation of root cells to form syncytia, are currently unknown.
US Patent Applications 20090077687 and 20090012029, identified nematode parasitism (effector) genes and described potential mechanisms to disrupt their expression and the function of their products to inhibit nematode parasitism of plants.